topic Re: Enabling SWO on MCXN947DK in MCX Microcontrollers

Enabling SWO on MCXN947DK

jcallon — Wed, 12 Nov 2025 17:56:11 GMT

Hello,

I'm trying to enable SWO on my MCXN947, but am having trouble getting it to output anything.

I have the SWO pin routed in MCUxpresso.
I have TRACE_CLK enabled, and set to half the speed of the main clock, because MCUxpresso does not let me set a TRACE_CLK speed above 96MHz .

I use DebugConsole_Init(0, 2000000, kSerialPort_Swo, 75000000); to initialize SWO. Because I have SDK_DEBUGCONSOLE=1U and SERIAL_PORT_TYPE_SWO=1U, this flow is called. It finishes without error.

File	Function
fsl_debug_console.c	DbgConsole_Init
fsl_debug_console.c	SerialManager_Init
fsl_component_serial_port_swo.c	Serial_SwoInit
fsl_debug_console.c	SerialManager_OpenWriteHandle
fsl_debug_console.c	SerialManager_OpenReadHandle

I use DebugConsole_PutChar('H') to try and output a byte on SWO, in an infinite loop. Because I have SDK_DEBUGCONSOLE=1U and SERIAL_PORT_TYPE_SWO=1U, this flow is called. It finishes without error.

File	Function
fsl_debug_console.c	DbgConsole_Putchar
fsl_debug_console.c	DbgConsole_SendDataReliable
fsl_component_serial_manager.c	SerialManger_WriteBlocking
fsl_component_serial_manager.c	SerialManger_Write
fsl_component_serial_manager.c	SerialManager_StartWriting
fsl_component_serial_port_swo.c	Serial_SwoWrite

After all this, I see no output on SWO. It is simply held high by my SEGGER J-Link.

Please let me know if there is some setup I am missing!
Otherwise, if someone has a working SWO setup on their own MCXN947DK, I'd really appreciate seeing your code and setup.

Thank you very much!
JC

Re: Enabling SWO on MCXN947DK

EdwinHz — Wed, 12 Nov 2025 20:22:07 GMT

Hi @jcallon,

For the proper procedure of executing SWO on our devices, please follow this guide: https://www.nxp.com/webapp/Download?colCode=MCUXPRESSO-SWO-TRACE&appType=license&location=null.

BR,
Edwin.

Re: Enabling SWO on MCXN947DK

jcallon — Wed, 12 Nov 2025 20:36:35 GMT

Thank you Edwin. I've seen that guide before, but I'll review it again in case I missed anything. It has many useful examples in the appendix for LPC* chips, but none I could fine for MCXN* chips.

Re: Enabling SWO on MCXN947DK

jcallon — Wed, 12 Nov 2025 23:06:33 GMT

Hi again,

To try and follow the posted guide, I've been trying to use SWO Trace Config to configure my SWO, instead of the DebugConsole_Init.

Load the frdmmcxn947_led_blinky_cm33_core0 example into the MXUxpresso IDE.
Mux SWO
My use case requires a 150MHz clock. Enable 150MHz clock by changing 'Initalize USBPHY clock' to 'PLL only'. Change BOARD_InitHardware(...) hardware_init.c to call BOARD_BootClockPLL150M(...) instead of BOARD_BootClockFRO12M(...), and update SysTick_Config(...) from to 12000000 to 150000000.
Set TRACECLK to 75 Mhz (because 96MHz is max).
Click Update Code.
Click Debug.
Click Resume All Debug sessions.
Open SWO Trace Config.
Click Change to configure main and trace clock speed.
Get an error. Are certain baud rates expected?

If you can, let me know if this issue reproduces on your side.
Thank you,
JC

Re: Enabling SWO on MCXN947DK

EdwinHz — Fri, 14 Nov 2025 21:52:41 GMT

Hi @jcallon,

I tried to replicate your issue with a FRDM-MCXN947, so I followed the same steps that you shared.

However, I noted something on step 3; You mentioned that your use case "requires a 150MHz clock. Enable 150MHz clock by changing 'Initalize USBPHY clock' to 'PLL only'."

That said, the frdmmcxn947_led_blinky_cm33_core0 example project already enables the main clock to be 150MHz, and the Functional Group is already BOARD_BootClockPLL150M. So, I didn't have to make any changes in the Clocks Tool, other than enabling the Trace Clock and setting it to 75MHz.

I didn't even have to enable the SWO pin, as it is also enabled on the example code already.

Are you using the latest release of the SDK?

On my side, I only had to do step 1, 4, 5, 6, 7, 8 and 9, and it worked correctly, without any error message regarding the baudrate.

Re: Enabling SWO on MCXN947DK

jcallon — Sat, 15 Nov 2025 00:51:46 GMT

Thank you Edwin.

I was using MCUxpresso 24.12, and updated to the latest I could find, 25.6.
Sure enough, when using the new MCUxpresso, like you said, the main clock was already configured 150MHz and SWO was already muxed.

Now, SWO trace config can find my board, at a clock speed of 150MHz, and a trace clock speed of 75MHz. I'm really happy MCUxpresso can find it now!

However, I did notice that after SWO Trace Config connected, the blinky on my board stopped running, and connection may not be complete. In screenshots from the MCUXpresso IDE Linkserver SWO Trace Guide, all dots on the right were green, for example, probe backend. Did you see this on your side?

Re: Enabling SWO on MCXN947DK

jcallon — Tue, 18 Nov 2025 00:50:24 GMT

An update, in case it helps someone out.

In my first post I mentioned using DbgConsole_Init(..., kSerialPort_Swo, ...) to initialize my SWO registers. Turns outs, all that was needed after that to make SWO be outputted for me was to enable the CoreSight Funnel Lock Access Register.

// https://developer.arm.com/documentation/100536/0302/About-the-programmers-model/ATB-funnel-registers/ATB-funnel-register-summary uint32_t *CSFUNNEL_LAR = (uint32_t *)(TPIU_BASE + 0x4000 + 0xFB0); *CSFUNNEL_LAR = 0xC5ACCE55;

I'm still interested in hearing how I can use MCUxpresso to set registers to get more interesting analytics over SWO than just printing characters, which is my current setup.

Re: Enabling SWO on MCXN947DK

jcallon — Tue, 18 Nov 2025 17:25:27 GMT

Oh, and I think I also needed to enable the ITM funnel for my port.
uint32_t *CSFUNNEL_CTRL = (uint32_t *)(TPIU_BASE + 0x4000);
*CSFUNNEL_CTRL |= 1; // Enable funnel for ITM (port 0)